The following should work nicely.

$(function() {

  // Way 1
  function doosomething()
  {
    //Doo something
  }

  // Way 2, equivalent to Way 1
  var doosomething = function() {
    // Doo something
  }

  $("div.class").click(doosomething);

  $("div.secondclass").click(doosomething);

});

Basically, you are declaring your function in the same scope as your are using it (JavaScript uses Closures to determine scope).

Now, since functions in JavaScript behave like any other object, you can simply assign doosomething as the function to call on click by using .click(doosomething);

Your function will not execute until you call it using doosomething() (doosomething without the () refers to the function but doesn't call it) or another function calls in (in this case, the click handler).